In general, the non-linear optical effect means a non-linear optical response proportional to the square, cube or higher power of photoelectric field applied. Known examples of the secondary non-linear optical effect proportional to the square of photoelectric field applied include second harmonic generation (SHG), optical rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing and light difference frequency mixing. Examples of the ternary non-linear optical effect proportional to the cube of photoelectric field applied include third harmonic generation (THG), optical Kerr effect, self-induced refractivity change and two-photon absorption.
As the non-linear optical material of exhibiting these non-linear optical effects, a large number of inorganic materials have been heretofore found. However, inorganic materials can be hardly used in practice because so-called molecular design so as to optimize desired non-linear optical characteristics or various properties necessary for the production of a device is difficult. On the other hand, organic compounds can realize not only optimization of desired non-linear optical characteristics by the molecular design but also control of other various properties and therefore, the probability of its practical use is high. Thus, organic compounds are attracting attention as a promising non-linear optical material.
In recent years, among non-linear optical characteristics of the organic compound, ternary non-linear optical effects, particularly, non-resonant two-photon absorption, are being taken notice of. The two-photon absorption is a phenomenon such that a compound is excited by simultaneously absorbing two photons. In the case where the two-photon absorption occurs in the energy region having no (linear) absorption band of the compound, this is called non-resonant two-photon absorption. In the following, even when not particularly specified, two-photon absorption indicates non-resonant two-photon absorption.
The non-resonant two-photon absorption efficiency is proportional to the square of photoelectric field applied (square-law characteristic of two-photon absorption). Therefore, when a laser is irradiated on a two-dimensional plane, two-photon absorption takes place only in the position having a high electric field strength at the center part of laser spot and utterly no two-photon absorption occurs in the portion having a weak electric field strength in the periphery. On the other hand, in a three-dimensional space, two-photon absorption occurs only in the region having a large electric field strength at the focus where the laser rays are converged through a lens, and two-photon absorption does not take place at all in the off-focus region because the electric field strength is weak. As compared with the linear absorption where excitation occurs in all positions proportionally to the strength of photoelectric field applied, in the non-resonant two-photon absorption, excitation occurs only at one point inside the space by virtue of the square-law characteristic and therefore, the space resolution is remarkably enhanced.
Usually, in the case of inducing non-resonant two-photon absorption, a short pulse laser in the near IR region having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present, and not having the absorption of the compound is used in many cases. Since a near IR ray in a so-called transparent region is used, the excitation light can reach the inside of a sample without being absorbed or scattered and one point inside the sample can be excited with very high space resolution due to the square-law characteristic of non-resonant two-photon absorption. The color change of a compound is, in other words, the change in the refractive index n of the real part and the extinction coefficient k of the imaginary part of the birefringence (n+ik) thereof. Accordingly, if dye precursor color-formation or dye absorption change can be induced in any desired point inside a three-dimensional space by the use of the excitation energy obtained through non-resonant two-photon absorption (or that is, if refractivity change can be induced), then the technology may be applied to three-dimensional recording mediums in which information data can be three-dimensionally written in a three-dimensional space, and to three-dimensional image display mediums in which image can be three-dimensionally displayed.
Optical information recording mediums and image display mediums using non-resonant two-photon absorbing compounds are disclosed in International Laid-Open WO97/09043. In the method disclosed in the publication, a polymer composition that contains a fluorescent two-photon absorbing dye is used for a recording medium and an image display medium, and a femtosecond pulse of Ti:sapphire laser is focused through a lens and radiated onto the recording medium or the image display medium to induce two-photon absorption at around the focus position, and the two-photon absorbing dye is thereby photolyzed and its fluorescent intensity is weakened. In the method, the difference between the thus-weakened fluorescent intensity and the strong fluorescent intensity of the non-irradiated part around the irradiated part is read out. However, the method requires strong light irradiation that causes the photolysis of the two-photon absorbing dye, and is therefore problematic in that the sensitivity of the method is low, and, in addition, since the fluorescent intensity change that radiates in all directions must be read out in the method, the contrast between the recorded area and the unrecorded area is low.
On the other hand, S. Kawata et al., Chem. Rev., Vol. 100, page 1777 (2000) reports an example of an optical recording medium on which information is recorded by inducing absorption change based on the two-photon photochromism of photochromic dyes. In this, however, since the cross-sectional area for two-photon absorption of the photochromic dyes used in the recording medium is small, the sensitivity of the recording medium is extremely low.
On the other hand, an optical information recording medium (optical disc) capable of recording information only once by laser light has been conventionally known and recordable CD (so-called CD-R), recordable DVD (so-called DVD-R) and the like are put into practical use.
For example, in a representative structure of DVD-R, a recording layer comprising a dye is provided on a disc-like substrate where guide grooves (pre-grooves) for tracking the laser light irradiated are formed at a pitch as narrow as a half or less (0.74 to 0.8 μm) of that in CD-R. On the recording layer, a light reflection layer is usually provided and if desired, a protective layer is further provided.
In the recording of information on DVD-R, visible laser light (usually in the range from 630 to 680 nm) is irradiated and absorbed in the irradiated portion of the recording layer, as a result, the temperature is locally elevated to cause physical or chemical changes (for example, production of pits) and in turn, changes in the optical characteristics, whereby the recording of information is effected. In the reading (reproduction) of information, laser light having the same wavelength as the laser light for recording is also irradiated and by detecting the difference in reflectance between the portion of the recording layer where the optical characteristics are changed (recorded area) and the portion where the optical characteristics are not changed (unrecorded area), the information is reproduced. This difference in reflectance is based on so-called “modulation of refractive index” and as the difference in refractive index between the recorded area and the unrecorded area is larger, the ratio of light reflectance, namely, S/N ratio in reproduction is advantageously larger.
Recently, network (e.g., Internet) and high-vision television are rapidly becoming widespread. In addition, the start of HDVT (High Definition Television) broadcasting is near at hand. To cope with such a tendency, demands for a large-capacity recording medium capable of easily and inexpensively recording image information of at least 50 GB, preferably at least 100 GB are increasing also in civilian uses.
Furthermore, in business uses such as use for backup of computer or broadcast, an optical recording medium capable of recording large-volume information of about 1 TB or more at high speed and low cost is being demanded.
However, in view of physical principle, conventional two-dimensional optical recording mediums such as DVD-R can have a capacity of about 25 GB at most even if the wavelength of light for recording/reproduction is shortened, and a recording capacity large enough to satisfy the requirement in future cannot be expected.
Under these circumstances, a three-dimensional optical recording medium is abruptly attracting an attention as an ultimate high-density, high-capacity recording medium. In the three-dimensional optical recording medium, recording is superposed in tens or hundreds of layers in the three-dimensional (thickness) direction to achieve super high-density and super high-capacity recording as large as tens or hundreds of times conventional two-dimensional recording mediums. In order to provide a three-dimensional optical recording medium, access and writing must be performed at any desired position in the three-dimensional (thickness) direction and as a technique therefor, a method of using a two-photon absorbing material and a method of using holography (interference) are known.
In the three-dimensional optical recording medium using a two-photon absorbing material, based on the above-described physical principle, so-called bit recording can be performed over tens or hundreds of times and a higher density recording can be attained. Thus, this is very an ultimate high-density high-capacity optical recording medium.
As for the three-dimensional optical recording medium using a two-photon absorbing material, for example, a method of using a fluorescent material for recording and reproduction and reading the information by fluorescence (see, JP-T-2001-524245 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”) by LEVICH, Eugene Boris et al., and JP-T-2000-512061 by PAVEL, Eugen, et al.), and a method of using a photochromic compound and reading the information by absorption or fluorescence (JP-T-2001-522119 by KOROTEEV, Nicolai I., et al., and JP-T-2001-508221 by ARSENOV, Vladimir, et al.) have been proposed. However, in either method, a specific two-photon absorbing material is not set forth and although examples of the two-photon absorbing compound are abstractly described, the two-photon absorbing compound used has a very small two-photon absorbing efficiency. In addition, these methods have a problem, for example, in the nondestructive reading, the long-term storability of record or the S/N ratio on reproduction and these systems are not practicable as an optical recording medium.
Particularly, in view of nondestructive reading, long-term record storage and the like, it is preferred to use an irreversible material and reproduce the information by detecting the change in reflectance (refractivity or absorbance). However, a two-photon absorbing material having such a function is not specifically disclosed in any publication.
Also, JP-A-6-28672 by Satoshi Kawada and Yoshimasa Kawada and JP-A-6-118306 by Satoshi Kawada, Yoshimasa Kawada et al. are disclosing, for example, an apparatus for three-dimensionally recording information by using the modulation of refractive index, and a reproducing apparatus and a reading method for the information, but a method using a two-photon absorbing three-dimensional optical recording material is not specifically described.
As described above, if reaction can be caused by using the excitation energy obtained upon non-resonant two-photon absorption and as a result thereof, the refractive index or the absorbance can be modulated between the laser-focused area and the unfocused area, then modulation of light reflectance due to modulation of refractive index or absorbance can be brought about at any desired position in a three-dimensional space with very high space resolution, and this enables application to a three-dimensional optical recording medium which is considered as an ultimate high-density recording medium. Furthermore, nondestructive reading can be achieved and because of an irreversible material, good storability can be expected, therefore, the practicability is high.
However, since two-photon absorbing compounds usable at present are low in the two-photon absorbing ability, a very high-output laser is necessary as the light source and the recording takes a long time.
In particular, for use in a three-dimensional optical recording medium, it is essential to establish a two-photon absorbing three-dimensional optical recording material capable of undergoing modulation of the refractive index or absorbance by two-photon absorption with high sensitivity. For this purpose, a material that contains a two-photon absorbing compound capable of absorbing two photons with high efficiency and producing an excited state, and a recording component capable of undergoing chemical reaction through electron transfer or energy transfer from the excited state of the two-photon absorbing compound to thereby record the refractivity difference or the absorbance difference may be useful. However, such a material has been heretofore not disclosed at all and establishment thereof is being demanded.